Braneworld models may yield interesting effects ranging from high-energy physics to cosmology, or even some low-energy physics. Their mode structure modifies standard results in these physical realms that can be tested and used to set bounds on the models parameters. Now, to define braneworld deviations from standard 4D physics, a notion of matter and gravity localization on the brane is crucial. In this work we investigate the localization of universal massive scalar fields in a de Sitter thick tachyonic braneworld generated by gravity coupled to a tachyonic bulk scalar field. This braneworld possesses a 4D de Sitter induced metric and is asymptotically flat despite the presence of a negative bulk cosmological constant, a novel and interesting peculiarity that contrasts with previously known models. Universal scalar fields can be localized in this expanding braneworld if their bulk mass obeys an upper bound, otherwise they delocalize: The dynamics of the scalar field is governed by a Schroedinger equation with an analog quantum mechanical potential of modified Poeschl-Teller type that depends on the bulk curvature of the braneworld system and the value of the bulk scalar field mass. For masses satisfying a certain upper bound, the potential displays a negative minimum and possesses a single massless bound state separated from the Kaluza-Klein (KK) massive modes by a mass gap defined by the Hubble (expansion scale) parameter of the 3-brane. As the bulk scalar field mass increases, the minimum of the quantum mechanical potential approaches a null value and, eventually, it becomes positive, transforming into a potential barrier and leading to delocalization of the bulk scalar field from the brane. The general solution of the Schroedinger equation is given in terms of general Heun functions, giving rise to a new application of these special functions.
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